Because of the trend towards higher attainable signal-to-noise ratio (SNR) with increasing magnetic field strength in Magnetic Resonance Imaging (MRI), magnet field strengths have increased to 9.4 Tesla state-of-the-art human research systems, and considerably higher for small animal systems. As the field strength increases, the Larmor frequency of the spin precession increases linearly with the relationship f(MHz)=42.6 MHz/Tesla. For MRI systems operated at approximately 3 Tesla (T), or above, the Larmor frequency is associated with wavelengths in the biological sample that are shorter than objects to be imaged. Accordingly, at magnetic field strengths of, for example, 3T and above, the homogeneity of transmit excitation in magnetic resonance imaging (MRI) is poor for samples the size of the human torso. This is a result of the standing waves created in the tissue by the coil conductors or array elements. This results in a somewhat new regime for very high field MRI. Heretofore, this issue has typically been addressed by optimizing the distribution of current on the coil conductors or array elements to create the best possible standing wave excitation pattern. The result has been that standard MRI methods extended to this short wavelength regime have produced undesirable outcomes (Collins et al., Magn Reson Med, 40: 847–856 (1998); Hoult et al., Magn Reson Imaging, 12: 46–67 (2000); Ibrahim et al, Magn Reson Imaging, 19: 219–226 (2001); Vaughan et al., Magn Reson Med, 46: 24–30 (2001). The relatively short wavelengths associated with standard MRI methods make it virtually impossible to produce an external source of spin excitation that has sufficient uniformity to obtain a relatively uniform image of the biological sample (Beck et al., Magn Reson Med, 51: 1103–1107 (2004); Hoult, D. I. J. Magn. Reson. Imaging 12: 46–67, (2000)).
Multiple receivers can create independence between receive channels by allowing phasing and amplitude adjustment to be performed arbitrarily after acquisition and using different effective coefficients for all positions. However, merely using multiple amplifiers or a power splitter does not create the needed independence of the local exciting elements because the local elements all transmit simultaneously so that the phase relationship between each element's field is fixed.
The standard assumption of optimization would be to drive multiple coils at prescribed amplitudes and phases, all with the same waveform. Additionally, transmit SENSE has been described and is designed to shorten the transmit period by using waveforms with different spectral characteristics.
Large B1 inhomogeneity can lead to non-uniform flip angles and non-uniform power absorption throughout the sample, and/or actual signal voids, or black areas, in the image. It is important to note that the images acquired in (Beck et al., Magn Reson Med, 51: 1103–1107 (2004)) were done with a multi-leg volume coil driven in quadrature, a fairly standard setup. In this case, the source of cancellations in the excitation field is due to the superposition of the fields of multiple sources around the object and that the fields have different phases as they extend across the sample.
Therefore, there is a need for a new technique for image creation for very high field magnetic resonance imaging systems.